Patent GB-519853-A (GB)
Title: Improvements in alternating current electric measuring and like instruments
IPC: G01R5/14
Filed: 1938-10-04
Abstract: 519,853. Current, voltage, power and phasedifference measurements. ELLIOTT BROS. (LONDON), Ltd., and SHOTTER, G. F. Oct. 4, 1938, No. 28779. [Class 37] [Also in Group XXXVII] An alternating-current instrument or relay includes a member of material having remanent magnetic properties which is acted on by magnetic fluxes displaced in phase and angularly displaced in space. The ammeter or voltmeter shown in Fig. 1 comprises two pairs of coils 1, la, 2, 2a clamped to blocks 3 mounted on a base 4, the axes of the two pairs being at right angles. The coils surround a disc 5 of steel or other material having remanent magnetic properties which is mounted on a spindle 6 rotatable in bearings 7, 8 and controlled by a spring 12. A laminated ring 13 of iron stampings is securea in a brass clamp 14 mounted on the base 4 and forms a return path for the fluxes set up by the coils. One pair of coils 1, 1a is connected between terminals 15, 16, Fig. 2, in series with an inductance L or condenser K, the other pair of coils 2, 2a being connected between the same terminals in series with a resistance R. Alternatively one pair of coils may be in series with a condenser and the other pair in series with an inductance and a resistance. In a modification, Fig. 5, three coils 20, 21, 22 are employed and may be curved as shown so as to embrace a greater portion of the disc 5. One end of each coil is connected to a common point, the other end of one coil 20 being connected to one terminal 15. The other ends of the coils 21, 22 are connected to the other terminal 16 through a resistance R and an inductance L or condenser K. The winding of the coil 20 is reversed and its turns are so chosen relatively to those of the coils 21, 22 that it produces a flux equal and opposite to the fluxes of the other two coils. The equivalent of a threephase field is thus produced, which reacts with the remanent magnetism of the disc to cause rotation. In another construction, Fig. 6, pairs of coils are wound on opposite poles 23, 23a, 24, 24a extending inwards from a common yoke 25. The phase displacement of the fluxes may be effected by shading rings on the pole tips. Each pole may be the central short limb of an E- shaped core having its longer limbs formed with inwardly directed extensions terminating close to the disc on opposite faces thereof, to form return paths for the fluxes. The instruments may be used as wattmeters, one pair of coils being connected in series with the load and the other pair across it, the reactances L, K being omitted. Phase differences may be indicated by connecting the pairs of coils to different supply sources. By connecting the free end of each coil of the instrument shown in Fig. 5 to one phase of a three-phase supply, phase sequence is indicated. In an alternative construction the member of magnetic material may be fixed and the magnetizing system movable. For heavy currents a single-turn coil may be formed of a conducting plate comprising two limbs 30, 31, Fig. 9, connected by a bridge part 32. A slot 33 embraces part of the disc 5, and slits 34 are cut in the outer edges of the limbs to increase the current path. Coils may be built up of several such plates. The Provisional Specification describes a modification in which the flux is produced by a cylindrical coil and a narrow fixed magnetizable bar arranged within the coil with its axis at an angle to the axis of the coil. The rotatable magnetic disc is mounted within the coil above the bar and lies in a plane parallel to the axis of the bar. When the exciting current has fallen to zero the remanent magnetism in the disc reacts with the remanent magnetism of the bar and causes movement of the disc.